Methods and apparatuses for transmitting control-plane messages in cells using different radio access technologies

ABSTRACT

A radio station (2) transmits or receives, to or from a radio terminal (1) in a second cell (23, 24), a CP message containing a NAS message or an RRC message or both, when a predetermined condition is satisfied. The second cell (23, 24) uses a RAT different from that of the first cell, and is used in addition and subordinate to the first cell. The predetermined condition relates to at least one of: (a) a content or type of the CP message; (b) a type of a signalling radio bearer used to transmit the CP message; (c) a transmission cause of the CP message; and (d) a type of a core network associated with the NAS message. It is thus, for example, possible to contributing to efficient transmission of control plane (CP) messages in a radio architecture that provides interworking of two different Radio Access Technologies (RATs).

The present application is a Continuation application of Ser. No.17/176,770 filed on Feb. 16, 2021, which is a Continuation applicationof Ser. No. 16/063,735 filed on Jun. 19, 2018, which issued as U.S. Pat.No. 10,952,099, which is a National Stage of International ApplicationNo. PCT/JP2016/087143 filed on Dec. 14, 2016, claiming priority based onJapanese Patent Application No. 2016-002879 filed on Jan. 8, 2016, thedisclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present disclosure relates to communication between a radio stationand a radio terminal using a plurality of Radio Access Technologies(RATs).

BACKGROUND ART

The 3rd Generation Partnership Project (3GPP) is starting to work on thestandardization for 5G, i.e., 3GPP Release 14, in 2016 to make 5G acommercial reality in 2020. 5G is expected to be realized by continuousenhancement/evolution of LTE and LTE-Advanced and an innovativedevelopment by an introduction of a new 5G air-interface (i.e., a newRadio Access Technology (RAT)). The new RAT (i.e., New 5G RAT) supports,for example, frequency bands higher than the frequency bands (e.g., 6GHz or lower) supported by the LTE/LTE-Advanced and itsenhancement/evolution. For example, the new RAT supports centimeter-wavebands (10 GHz or higher) and millimeter-wave bands (30 GHz or higher).

Higher frequency can provide higher-rate communication. However, becauseof its frequency properties, coverage of the higher frequency is morelocal. Therefore, high frequencies are used to boost capacity and datarates in specific areas, while wide-area coverage is provided by lowercurrent frequencies. That is, in order to ensure the stability of New 5GRAT communication in high frequency bands, tight integration orinterworking between low and high frequencies (i.e., tight integrationor interworking between LTE/LTE-Advanced and New 5G RAT) is required. A5G supporting radio terminal (i.e., 5G User Equipment (UE)) is connectedto both of a low frequency band cell and a high frequency band cell(i.e., a LTE/LTE-Advanced cell and a new 5G cell) by using CarrierAggregation (CA) or Dual Connectivity (DC), or a modified techniquethereof.

Non-Patent Literature 1 discloses user-plane and control-planearchitectures to use both the LTE air interface (i.e., LTE RAT) and thenew 5G air interface (i.e., New 5G RAT). In some implementations, acommon Radio Resource Control (RRC) layer and a common Packet DataConvergence Protocol (PDCP) layer (or sublayer) are used. The commonPDCP layer is connected to LTE lower layers and New 5G lower layers, andprovides an upper layer with a transfer service of user plane data andcontrol plane data through the LTE lower layers and the New 5G lowerlayers. The LTE lower layers include a Radio Link Control (RLC) layer, aMedium Access Control (MAC) layer, and a physical layer for the LTE-RAT.In a similar way, the New 5G lower layers include an RLC layer, a MAClayer, and a physical layer for the New 5G RAT.

Non-Patent Literature 1 further discloses transmitting the samecontrol-plane message simultaneously on both an LTE cell and a New 5Gcell (i.e., Control Plane Diversity) and switching the path of acontrol-plane connection from an LTE cell to a New 5G cell and viceversa (i.e., Fast Control Plane Switching) based on the premise that thecommon PDCP layer and the common RRC layer are arranged in the LTE basestation. The use of the common PDCP and the common RRC layer allows theUE to be connected to one control point (i.e., the common RRC layer) viaone of a plurality of air interfaces.

The term “LTE” used in this specification includes enhancements of LTEand LTE-Advanced for 5G to provide tight interworking with the New 5GRAT, unless otherwise indicated. Such enhancements of LTE andLTE-Advanced are also referred to as LTE-Advanced Pro, LTE +, orenhanced LTE (eLTE). Further, the term “5G” or “New 5G” in thisspecification is used, for the sake of convenience, to indicate anair-interface (RAT) that is newly introduced for the fifth generation(5G) mobile communication systems, and nodes, cells, protocol layers,etc. related to this air-interface. The names of the newly introducedair interface (RAT), and nodes, cells, and protocol layers relatedthereto will be determined in the future as the standardization workprogresses. For example, the LTE RAT may be referred to as Primary RAT(P-RAT or pRAT) or Master RAT. Meanwhile, the New 5G RAT may be referredto as Secondary RAT (S-RAT or sRAT).

CITATION LIST Non Patent Literature

[Non-Patent Literature 1] Da Silva, I.; Mildh, G.; Rune, J.; Wallentin,P.; Vikberg, J.; Schliwa-Bertling, P.; Rui Fan, “Tight Integration ofNew 5G Air Interface and LTE to Fulfill 5G Requirements,” in VehicularTechnology Conference (VTC Spring), 2015 IEEE 81st, pp.1-5, 11-14 May2015

SUMMARY OF INVENTION Technical Problem

The inventors have studied the 5G radio architecture that provides tightinterworking of the LTE RAT and the New 5G RAT and found some problems.For example, Non-Patent Literature 1 does not take into account acontrol plane specific to New 5G cells. In some implementations, controlplane (CP) messages specific to New 5G cells (e.g., allocation of atemporary UE ID, configuration of lower layers, signalling for servicequality management, and measurement reporting) may be required. In someimplementations, a signalling connection specific to 5G cells (e.g., anRRC connection and a Signaling Radio Bearer (SRB)) may be required. Itmay be efficient for the New-5G-cell-specific control plane (CP)messages and the 5G-cell-specific signalling connections to betransmitted via New 5G cells, not via LTE cells. For example, one of theimportant requirements for 5G is low latency, and accordingly it can beexpected that 5G UEs will be able to perform communication on New 5Gcells with latency lower than that of communication on LTE cells.

Accordingly, one of the objects to be attained by embodiments disclosedherein is to provide an apparatus, a method, and a program thatcontribute to efficient transmission of control-plane messages through acell of the subordinate RAT in a radio architecture that providesinterworking of two different RATs. It should be noted that this objectis merely one of the objects to be attained by the embodiments disclosedherein. Other objects or problems and novel features will be madeapparent from the following description and the accompanying drawings.

Solution to Problem

In a first aspect, a radio station system includes one or more radiostations. The one or more radio stations are configured tosimultaneously provide, for one radio terminal, at least one first cellin accordance with a first radio access technology and at least onesecond cell in accordance with a second radio access technology and usedin addition and subordinate to the at least one first cell. The one ormore radio stations are configured to transmit or receive acontrol-plane message to or from the radio terminal on the at least onesecond cell when a predetermined condition is satisfied. Thecontrol-plane message includes a non-access stratum (NAS) message or aradio resource control (RRC) message or both. The predeterminedcondition relates to at least one of: (a) a content or type of thecontrol-plane message; (b) a type of a signalling radio bearer used totransmit the control-plane message; (c) a transmission cause of thecontrol-plane message; and (d) a type of a core network associated withthe NAS message.

In a second aspect, a method in a radio station system, including one ormore radio stations, includes:

-   (a) simultaneously providing, for one radio terminal, at least one    first cell in accordance with a first radio access technology and at    least one second cell in accordance with a second radio access    technology and used in addition and subordinate to the at least one    first cell; and-   (b) transmitting or receiving a control-plane message to or from the    radio terminal on the at least one second cell when a predetermined    condition is satisfied. The control-plane message includes a    non-access stratum (NAS) message or a radio resource control (RRC)    message or both. The predetermined condition relates to at least one    of: (a) a content or type of the control-plane message; (b) a type    of a signalling radio bearer used to transmit the control-plane    message; (c) a transmission cause of the control-plane message;    and (d) a type of a core network associated with the NAS message.

In a third aspect, a radio terminal includes a memory and at least oneprocessor coupled to the memory. The at least one processor isconfigured to communicate with a radio station system including one ormore radio stations simultaneously on at least one first cell inaccordance with a first radio access technology and at least one secondcell in accordance with a second radio access technology and used inaddition and subordinate to the at least one first cell. The at leastone processor is configured to transmit or receive a control-planemessage to or from the radio station system on the at least one secondcell when a predetermined condition is satisfied. The control-planemessage includes a non-access stratum (NAS) message or a radio resourcecontrol (RRC) message or both. The predetermined condition relates to atleast one of: (a) a content or type of the control-plane message; (b) atype of a signalling radio bearer used to transmit the control-planemessage; (c) a transmission cause of the control-plane message; and (d)a type of a core network associated with the NAS message.

In a fourth aspect, a method in a radio terminal includes:

-   (a) communicating with a radio station system including one or more    radio stations simultaneously on at least one first cell in    accordance with a first radio access technology and at least one    second cell in accordance with a second radio access technology and    used in addition and subordinate to the at least one first cell; and-   (b) transmitting or receiving a control-plane message to or from the    radio station system on the at least one second cell when a    predetermined condition is satisfied. The control-plane message    includes a non-access stratum (NAS) message or a radio resource    control (RRC) message or both. The predetermined condition relates    to at least one of: (a) a content or type of the control-plane    message; (b) a type of a signalling radio bearer used to transmit    the control-plane message; (c) a transmission cause of the    control-plane message; and (d) a type of a core network associated    with the NAS message.

In a fifth aspect, a program includes instructions (software codes)that, when loaded into a computer, cause the computer to perform themethod according to the aforementioned second or fourth aspect.

Advantageous Effects of Invention

According to the above-described aspects, it is possible to provide anapparatus, a method, and a program that contribute to efficienttransmission of control-plane messages via a cell of the subordinate RATin a radio architecture that provides interworking of two differentRATs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a configuration example of a radiocommunication network according to several embodiments;

FIG. 2 is a diagram showing a configuration example of the radiocommunication network according to the several embodiments;

FIG. 3 is a diagram showing another configuration example of the radiocommunication network according to the several embodiments;

FIG. 4 is a diagram showing an example of a radio protocol stackaccording to the several embodiments;

FIG. 5 is a diagram showing an example of the radio protocol stackaccording to the several embodiments;

FIG. 6 is a diagram showing an example of a layer-2 structure accordingto the several embodiments;

FIG. 7 is a sequence diagram showing one example of operations of aradio terminal and a base station according to a first embodiment;

FIG. 8 is a sequence diagram showing one example of operations of theradio terminal and the base station according to the first embodiment;

FIG. 9 is a sequence diagram showing one example of operations of theradio terminal and the base station according to the first embodiment;

FIG. 10 is a flowchart showing one example of operations of an RRC layeraccording to the first embodiment;

FIG. 11 is a table showing an example of a key used to generate atemporary key for ciphering/deciphering of each radio bearer accordingto a second embodiment;

FIG. 12 is a block diagram showing a configuration example of a radioterminal according to the several embodiments; and

FIG. 13 is a block diagram showing a configuration example of a basestation according to the several embodiments.

DESCRIPTION OF EMBODIMENTS

Specific embodiments are described hereinafter in detail with referenceto the drawings. The same or corresponding elements are denoted by thesame reference symbols throughout the drawings, and repetitivedescriptions are avoided for clarity.

Each of embodiments described below may be used individually, or two ormore of the embodiments may be appropriately combined with one another.These embodiments include novel features different from one another.Accordingly, these embodiments contribute to achieving objects orsolving problems different from one another and contribute to obtainingadvantages different from one another.

The following descriptions on the embodiments mainly focus on specificexamples with regard to the 5G radio architecture that provides tightinterworking of the LTE RAT and the New 5G RAT. However, theseembodiments are not limited to being applied to the 5G radioarchitecture and may also be applied to other radio architectures thatprovide tight interworking of two different RATs.

First Embodiment

FIG. 1 shows a configuration example of a radio communication networkaccording to several embodiments including this embodiment. In theexample shown in FIG. 1 , the radio communication network includes aradio terminal (UE) 1 and an integrated base station (i.e., integratedeNB) 2. The UE 1 is a 5G UE and connects to both one or more LTE cells(e.g., cells 21 and 22) and one or more New 5G cells (e.g., cells 23 and24) using CA, DC, or an enhancement thereof. In the followingdescription, one or more LTE cells are referred to as an LTE cell group(CG) and one or more New 5G cells used by the 5G UE 1 are referred to asa New 5G CG. Each of the cells in the LTE CG and New 5G CG has beenconfigured for the 5G UE 1 by the integrated eNB 2 and has beenactivated by the integrated eNB 2. In some implementations, frequencybands (e.g., F1 and F2) of the LTE CG (e.g., the cells 21 and 22) arelower frequency bands (e.g., lower than 6 GHz) and frequency bands(e.g., F3 and F4) of the New 5G CG (e.g., the cells 23 and 24) arehigher frequency bands (e.g., higher than 6 GHz).

The integrated eNB 2 supports 5G and provides a plurality of cells thatuse a plurality of component carriers (CCs) having different frequenciesand using different RATs. In the example shown in FIG. 1 , theintegrated eNB 2 provides LTE cells 21 and 22 and New 5G cells 23 and24. The integrated eNB 2 communicates with the 5G UE 1 via both the LTECG (e.g., the cells 21 and 22) and the New 5G CG (e.g., the cells 23 and24) using CA, DC, or an enhancement thereof. Further, the integrated eNB2 is connected to a core network, that is, an integrated Evolved PacketCore (i.e., integrated EPC) 41. The integrated EPC 41 provides LTE corenetwork functions and 5G new core network functions. In someimplementations, the integrated eNB 2 may be connected to a 5G specificcore network (i.e., 5G specific EPC 42).

As shown in FIG. 2 , a remote radio unit 3 may be used to provide atleast one of the cells of the integrated eNB 2 (e.g., New 5G cells 23and 24). In the configuration shown in FIG. 2 , the integrated eNB 2performs digital signal processing regarding uplink and downlinksignals, and meanwhile the radio unit 3 performs analog signalprocessing of the physical layer. For example, the integrated eNB 2 andthe radio unit 3 are connected to each other by an optical fiber, and adigital baseband signal is transferred through this optical fiber inaccordance with the Common Public Radio Interface (CPRI) standard. Theconfiguration shown in FIG. 2 is referred to as a Cloud Radio AccessNetwork (C-RAN). The radio unit 3 is referred to as a Remote Radio Head(RRH) or a Remote Radio Equipment (RRE). The integrated eNB 2 thatperforms baseband digital signal processing is referred to as a BasebandUnit (BBU). Further, information about any one of the layers 1, 2, and 3(or a signal containing this information) may be transferred using afronthaul (interface) that is to be standardized by, for example, 3GPPor Small Cell Forum. For example, a form in which the fronthaul connectsbetween the L1 and the L2 or between Sub-layers in the L2 is alsoreferred to as L2 C-RAN. In this case, the integrated eNB 2 and the RRH3 shown in FIG. 2 are also referred to as a Digital Unit (DU) and aRadio Unit (RU), respectively.

In the configuration examples shown in FIGS. 1 and 2 , the LTE radioprotocol and the New 5G radio protocol are implemented in one node(i.e., the integrated eNB 2). Accordingly, the configuration examplesshown in FIGS. 1 and 2 are referred to as co-located deployments orco-located RAN. In the case of the L2 C-RAN configuration, a part of theNew 5G radio protocol may be deployed in the RU. However, in anotherconfiguration example, non co-located deployments or non co-located RANmay be employed. In the Non co-located deployments, the LTE radioprotocol and the New 5G radio protocol are provided by two nodes (eNBs)different from each other. These two nodes are installed, for example,at two different sites geographically spaced apart from each other.

FIG. 3 shows an example of the non co-located deployments of the radiocommunication network according to several embodiments including thisembodiment. In the example shown in FIG. 3 , the radio communicationnetwork includes a 5G UE 1, an LTE+ eNB 5, and a 5G specific eNB 6. TheLTE+ eNB 5 provides an LTE CG (e.g., the cells 21 and 22) and the 5Gspecific eNB 6 provides a New 5G CG (e.g., the cells 23 and 24). TheLTE+ eNB 5 is connected to the 5G specific eNB 6 by a communicationline, such as an optical fiber link or a point-to-point radio link, andcommunicates with the 5G specific eNB 6 on an inter-base-stationinterface 301 (e.g., enhanced X2 interface). The LTE+ eNB 5 and the 5Gspecific eNB 6 interwork with each other to enable the 5G UE 1 toconnect to both the LTE CG and the 5G CG using CA, DC, or an enhancementthereof.

FIG. 4 shows one example of the radio protocol stack supported by the 5GUE 1 and the integrated eNB 2. A radio protocol stack 400 shown in FIG.4 includes a unified (or integrated) RRC layer 401 and a unified (orintegrated) PDCP layer (or sublayer) 402. The integrated RRC layer 401and the integrated PDCP layer 402 may also be referred to as a commonRRC layer and a common PDCP layer, respectively. The radio protocolstack 400 further includes LTE lower layers and New 5G lower layers. TheLTE lower layers include an LTE RLC layer 403, an LTE MAC layer 404, andan LTE PHY layer 405. The New 5G lower layers include a New RLC layer406, a New MAC layer 407, and a New PHY layer 408. In the case of usingthe integrated eNB 2, some of the functions of the LTE PHY layer 405(e.g., analog signal processing) may be provided by an RRH for LTE. In asimilar way, some of the functions of the New PHY layer 408 (e.g.,analog signal processing) may be provided by an RRH for New 5G. Further,in the case of using the above-described L2 C-RAN configuration, some ofthe functions of the New PHY layer, the New MAC layer, or the New RLClayer (and the functions of layers lower than it) may be provided by anRU for New 5G.

The integrated RRC layer 401 provides control-plane functions in the LTERAT and the New 5G RAT. The main services and functions provided by theintegrated RRC layer 401 include the following:

-   Transmission of system information for non-access stratum (NAS) and    access stratum (AS);-   Paging;-   Establishment, maintenance, and release of RRC connections;-   Security functions including key management;-   Configuration, maintenance, and release of radio bearers;-   Configuration of lower layer protocols (i.e., PDCP, RLC, MAC, and    PHY);-   QoS management;-   UE measurement report and configuration thereof; and-   Transfer of NAS messages between a UE and a core network.

The integrated RRC layer 401 communicates with the integrated PDCP layer402 to perform management of radio bearers, control ofciphering/deciphering of data of the user plane (i.e., data radiobearers), control of ciphering/deciphering of data (i.e., RRC PDUs) ofthe control plane (i.e., signalling radio bearers), and control ofintegrity protection of data (i.e., RRC PDUs) of the control plane(i.e., signalling radio bearers). Further, the integrated RRC layer 401controls the LTE RLC layer 403, the LTE MAC layer 404, and the LTE PHYlayer 405, and also controls the New RLC layer 406, the New MAC layer407, and the New PHY layer 408.

The integrated PDCP layer 402 provides an upper layer with transferservices of data of data radio bearers and signalling radio bearers. Theintegrated PDCP layer 402 receives services from the LTE RLC layer 403and the New RLC layer 406. That is, the integrated PDCP layer 402 isprovided with a transfer service of PDCP PDUs through the LTE RAT by theLTE RLC layer 403 and is provided with a transfer service of PDCP PDUsthrough the New 5G RAT by the New RLC layer 406.

It should be noted that the radio protocol stack 400, which uses theintegrated PDCP layer 402, shown in FIG. 4 can be applied not only tothe co-located deployments (e.g., FIGS. 1 and 2 ) but also to the nonco-located deployments (e.g., FIG. 3 ). That is, as shown in FIG. 5 , inthe non co-located deployments, the LTE+ eNB 5 is arranged in a site 501and provides the integrated RRC layer 401, the integrated PDCP layer402, the LTE RLC layer 403, the LTE MAC layer 404, and the LTE PHY layer405. In contrast, the 5G specific eNB 6 is arranged in another site 502and provides the New RLC layer 406, the New MAC layer 407, and the NewPHY layer 408.

In some implementations, the 5G specific eNB 6 used in the nonco-located deployments may include a New RRC layer 511 and a New PDCPlayer 512. Further, the 5G specific eNB 6 may include a controlinterface or connection (e.g., an S1-MME interface or an S1-U interface)with a core network (e.g., the integrated EPC 41 or the 5G specific EPC42) for the 5G UE 1. In some implementations, the New RRC layer 511 mayconfigure the lower layers 406-408 of the New 5G CG (e.g., New 5G cells23 and 24) and transmit system information (i.e., Master InformationBlock (MIB), or System Information Blocks (SIBs), or both) via the New5G CG. The New RRC layer 511 may configure a signalling radio bearerwith the 5G UE 1, also configure the lower layers 406-408 of the New 5GCG (e.g., the New 5G cells 23 and 24) and the New PDCP layer 512, andthen transmit or receive RRC messages to or from the 5G UE 1 through theNew 5G CG. The New RRC layer 511 may transfer NAS messages between thecore network (e.g., the integrated EPC 41 or the 5G specific EPC 42) andthe 5G UE 1. The New PDCP layer 512 provides the New RRC layer 511 witha transfer service of RRC messages via the New 5G lower layers 406-408.

The New RRC layer 511 may depend on the integrated RRC layer 401 (i.e.,have a dependency relationship) or may perform control similar to thatperformed by the integrated RRC layer 401 (i.e., have a similarfunction). In the former case (i.e., dependency relationship), the 5Gspecific eNB 6 (or the New RRC layer 511 thereof) may generate RRCconfiguration information with respect to a New 5G cell(s) (i.e., New 5GCG) in response to an instruction or a request from the LTE+ eNB 5 (orthe integrated RRC layer 401 thereof). The 5G specific eNB 6 (or the NewRRC layer 511 thereof) may transmit this RRC configuration informationto the LTE+ eNB 5 (or the integrated RRC layer 401 thereof) and the LTE+eNB 5 may transmit an RRC message containing this RRC configurationinformation (e.g., an RRC Connection Reconfiguration message) to the 5GUE 1 on an LTE cell (i.e., LTE CG). Alternatively, the 5G specific eNB 6(or the New RRC layer 511 thereof) may transmit an RRC messagecontaining this RRC configuration information to the 5G UE 1 on a New 5Gcell.

The 5G UE 1 may support the protocol stack shown in FIG. 4 or supportanother protocol stack to communicate with the radio network shown inFIG. 5 . For example, the 5G UE 1 may have an RRC layer (i.e., a masterRRC layer or a primary RRC layer) corresponding to the integrated RRClayer 401 of the LTE+ eNB 5 and an auxiliary RRC layer (i.e., a sub RRClayer or a secondary RRC layer) corresponding to the New RRC layer 511of the 5G specific eNB 6. For example, the sub RRC layer may perform oneor both of transmission and reception (or one or both of generation andrestoration) of a part of the RRC configuration information controlledby the master RRC layer. The 5G UE 1 may receive both the RRCconfiguration information regarding a LTE cell(s) (i.e., LTE CG) and theRRC configuration information regarding a New 5G cell(s) (i.e., New 5GCG) through an LTE cell or through a New 5G cell. Alternatively, the 5GUE 1 may receive the RRC configuration information regarding a LTEcell(s) (i.e., LTE CG) through an LTE cell and meanwhile receive the RRCconfiguration information regarding a New 5G cell(s) (i.e., New 5G CG)through a New 5G cell.

The radio protocol stack shown in FIG. 4 is merely one example and,alternatively, the 5G UE 1 and the integrated eNB 2 may support anotherprotocol stack. For example, in FIG. 4 , the integrated PDCP layer 402integrates (or allows interworking of) the LTE lower layers and the New5G lower layers. Alternatively, an integrated MAC layer may be used tointegrate (or allow interworking of) the LTE PHY layer 405 and the NewPHY layer 408.

FIG. 6 shows one example of the layer-2 structure for uplink accordingto the several embodiments. The layer-2 structure for downlink issimilar to that shown in FIG. 6 except for some points, such as theterms used to describe the transport channels. An integrated PDCP layer602, an LTE RLC layer 603, an LTE MAC layer 604, a New RLC layer 606,and a New MAC layer 607 shown in FIG. 6 respectively correspond to theintegrated PDCP layer 402, the LTE RLC layer 403, the LTE MAC layer 404,the New RLC layer 406, and the New MAC layer 407 shown in FIGS. 4 and 5.

The integrated PDCP layer 602 includes one or more PDCP entities. EachPDCP entity transports data of one radio bearer. Each PDCP entity isassociated with either the user plane or the control plane depending onwhich radio bearer (i.e., a data radio bearer (DRB) or a signallingradio bearer (SRB)) it transports data from. In the example shown inFIG. 6 , the integrated PDCP layer 602 includes three PDCP entities6021, 6022, and 6023 that correspond to three radio bearers #1, #2, and#3, respectively. Each of the radio bearers #1, #2, and #3 may be an SRBor a DRB.

The data of the radio bearer #1 is transmitted from the 5G UE 1 to theintegrated eNB 2 (or the LTE+ eNB 5) via the LTE RAT on the LTE CG(e.g., the LTE cells 21 and 22). Accordingly, the radio bearer #1 may behereinafter referred to as an LTE bearer. The radio bearer #1 is similarto an MCG bearer in LTE Release 12 DC.

The data of the radio bearer #2 is transmitted from the 5G UE 1 to theintegrated eNB 2 (or the 5G specific eNB 6) via the New 5G RAT on theNew 5G CG (e.g., the New 5G cells 23 and 24). Accordingly, the radiobearer #2 may be hereinafter referred to as a New 5G bearer. When thedata is transmitted on the New 5G CG managed by the 5G specific eNB 6,the radio bearer #2 is similar to an SCG bearer in LTE Release 12 DC.Alternatively, when the data is transmitted on the New 5G CG managed bythe integrated eNB 2, the radio bearer #2 may be similar to a bearer onthe SCG side of a split bearer in LTE Release 12 DC.

The radio bearer #3 is similar to a split bearer in LTE Release 12 DC.That is, the radio bearer #3 is associated with both of one logicalchannel of the LTE RAT and one logical channel of the New 5G RAT to useboth the resources of the LTE CG and the resources of the New 5G CG. Inthe case of the user data, the logical channel of the LTE RAT is aDedicated Traffic Channel (DTCH). The logical channel of the New 5G RATis a 5G logical channel for the user data that corresponds to the DTCH.The radio bearer #3 may be hereinafter referred to as a split bearer ora unified bearer (an integrated bearer).

In the case of uplink transmission by the 5G UE 1, the PDCP entity 6021generates PDCP PDUs from data of the radio bearer #1 (i.e., LTE bearer)and sends these PDCP PDUs to an LTE RLC entity 6031. In the case ofuplink reception by the integrated eNB 2 (or the LTE+ eNB 5), the PDCPentity 6021 receives RLC SDUs (i.e., PDCP PDUs) from the LTE RLC entity6031 and sends data of the radio bearer #1 to the upper layer.

In the case of uplink transmission by the 5G UE 1, the PDCP entity 6022generates PDCP PDUs from data of the radio bearer #2 (i.e., New 5Gbearer) and sends these PDCP PDUs to a New RLC entity 6061. In the caseof uplink reception by the integrated eNB 2 (or the 5G specific eNB 6),the PDCP entity 6022 receives RLC SDUs (i.e., PDCP PDUs) from the NewRLC entity 6061 and sends data of the radio bearer #2 to the upperlayer.

In the case of uplink transmission by the 5G UE 1, the PDCP entity 6023generates PDCP PDUs from data of the radio bearer #3 (i.e., integratedbearer) and routes these PDCP PDUs to an LTE RLC entity 6032 or a NewRLC entity 6062. In the case of uplink reception by the integrated eNB 2(or the LTE+ eNB 5 and the 5G specific eNB 6), the PDCP entity 6023reorders PDCP PDUs (i.e., RLC SDUs) received from the LTE RLC entity6032 and the New RLC entity 6062 and sends data of the radio bearer #3to the upper layer.

Each RLC entity in the LTE RLC layer 603 and the New RLC layer 606 isconfigured, by the integrated RRC entity (i.e., the RRC entity 401 shownin FIG. 4 ), with RLC Acknowledged Mode (RLC AM) data transfer or RLCUnacknowledged Mode (RLC UM) data transfer, and then provides a transferservice of PDCP PDUs. In the case of uplink transmission by the 5G UE 1,each RLC entity in the LTE RLC layer 603 generates RLC PDUs (i.e., dataof one logical channel) from PDCP PDUs (i.e., RLC SDUs) and sends theseRLC PDUs to a MAC entity 6041 in the LTE MAC layer 604. In a similarway, each RLC entity in the New RLC layer 606 generates RLC PDUs (i.e.,data of one logical channel) from PDCP PDUs (i.e., RLC SDUs) and sendsthem to a MAC entity 6071 in the New MAC layer 607.

In the example shown in FIG. 6 , one MAC entity 6041 is used for two LTEcells (i.e., LTE CG) configured for one 5G UE 1. In the case of uplinktransmission by the 5G UE 1, the MAC entity 6041 multiplexes RLC PDUs(i.e., MAC SDUs), which belong to the two logical channels from the twoRLC entities 6031 and 6032, into two transport blocks per TransmissionTime Interval (TTI). The two transport blocks per TTI are sent to theLTE physical layer 405 through two UL transport channels (i.e., UL-SCHs)corresponding to the two LTE cells 21 and 22.

In a similar way, one MAC entity 6071 is used for two New 5G cells(i.e., New 5G CG) configured for one 5G UE 1. In the case of uplinktransmission by the 5G UE 1, the MAC entity 6071 multiplexes RLC PDUs(i.e., MAC SDUs), which belong to two logical channels from two RLCentities 6071 and 6072, into two transport blocks per Transmission TimeInterval (TTI). The two transport blocks per TTI are sent to thephysical layer 408 for New 5G through two UL transport channels (i.e.,UL TrCHs) corresponding to the two New 5G cells 23 and 24.

As described above, the layer-2 structure for downlink is similar tothat shown in FIG. 6 except for some points, such as the terms used todescribe the transport channels. For example, in the case of downlinkreception by the 5G UE 1, the PDCP entity 6023 of the 5G UE 1 reordersPDCP PDUs (i.e., RLC SDUs) received from the LTE RLC entity 6032 andfrom the New RLC entity 6062, and then sends data of the radio bearer #3(i.e., integrated bearer) to the upper layer.

In the following, an operation of transmitting a control-plane (CP)message performed by the 5G UE 1 and the integrated eNB 2 (or the LTE+eNB 5 and the 5G specific eNB 6) according to this embodiment will beexplained. The 5G UE 1 and the base station system are configured totransmit CP messages (i.e., RRC signalling) on a New 5G cell aftercompletion of configuration and activation of this New 5G cell (i.e.,after the New 5G cell becomes available for the 5G UE 1). The 5G UE 1and the base station system may switch the cell, to be used to transmitCP messages, from the LTE cell to the New 5G cell. Further oralternatively, the 5G UE 1 and the base station system may adaptivelychange the cell, to be used to transmit CP messages, between the LTEcell and the New 5G cell. The CP messages include NAS messages or RRCmessages or both. The base station system is the integrated eNB 2, or acombination of the LTE+ eNB 5 and the 5G specific eNB 6.

To be more specific, the 5G UE 1 is configured to transmit or receive CPmessages to or from the base station system on any New 5G cell withinthe New 5G CG when a predetermined condition is satisfied. In a similarway, the base station system is configured to transmit or receive CPmessage to or from the 5G UE 1 on any New 5G cell within the New 5G CGwhen a predetermined condition is satisfied. The predeterminedcondition, which is used to determine which one of the LTE CG and theNew 5G CG is used to transmit a CP message, relates to at least one ofthe following:

-   (a) the content or type of the CP message-   (b) the type of the signalling radio bearer used to transmit the CP    message,-   (c) the transmission cause of the CP message; and-   (d) the type of the core network associated with the NAS message.

The above condition (a) may be that the content or type of the CPmessage is (or is not) a specific content or type. The specific contentor type may be at least one of: a request for terminal measurementregarding the New 5G cell; a report on a result of the measurementregarding the New 5G cell; a request for terminal (holding) informationheld by the terminal; a report on the terminal information; securityconfiguration information about the access stratum (AS) of the New 5Gcell; a request for AS-security activation; a response to the requestfor AS-security activation; configuration information specific to theNew 5G RAT; and control information that will be newly defined for theNew 5G cell by the 3GPP.

The above condition (b) may be that the type of the signalling radiobearer used to transmit the CP message is (or is not) a specific type.For example, the specific signalling-radio-bearer type may be a typecorresponding to SRB0, SRB1, or SRB2 in LTE specified in advance, or maybe a type corresponding to a new signalling radio bearer (e.g., SRBx).

The above condition (c) may be that the transmission cause of the CPmessage is (or is not) a specific transmission cause. For example, thespecific transmission cause may be at least one of: degradation of aradio quality of the New 5G cell; a failure of a radio linkestablishment in the LTE cell or the New 5G cell; a request forre-establishment of the radio link after the failure of the radio linkestablishment; and a response to the request for re-establishment of theradio link.

The above condition (d) may be that the type of the core network usedfor the transmission of the NAS message is (or is not) a specificcore-network type. For example, the specific core-network type may be atype corresponding to a core network that will be newly introduced forthe New 5G RAT or may be a type corresponding to a core network that ispreviously designated for use for a specific service or function.

In a first implementation, the 5G UE 1 may determine which one of theNew 5G CG and the LTE CG is to be used to transmit an RRC messagedepending on whether this RRC message includes a reporting message fromthe 5G UE 1 indicating a measurement result of one or more New 5G cellswithin the New 5G CG (i.e., Measurement report message). That is, whenan RRC message includes a report from the 5G UE 1 on a measurementresult of one or more New 5G cells, the 5G UE 1 may transmit this RRCmessage on any cell within the New 5G CG. The base station system mayreceive this RRC message on any cell within the New 5G CG. In contrast,when an RRC message includes a report from the 5G UE 1 on a measurementresult of one or more LTE cells, the 5G UE 1 may transmit this RRCmessage on any LTE cell. The report on the measurement result to betransmitted on the New 5G CG may be distinguished from the report on themeasurement result to be transmitted on the LTE CG, on the basis of ameasurement instruction (e.g., MeasConfig IE) or a criterion defining acondition to trigger a measurement report event (e.g., ReportConfigEUTRAIE). For example, a measurement report event, where a cell (e.g.,PSCell, special cell, or SCell) within the New 5G CG is a target, isdefined. In this case, the 5G UE 1 may transmit a report on ameasurement result triggered by this measurement report event on anycell within the New 5G CG. The measurement report event, where a cellwithin the New 5G CG is a target, may be defined as Inter-RATmeasurement with respect to a LTE cell.

In a second implementation, the 5G UE 1 may determine which one of theNew 5G CG and the LTE CG is to be used to transmit an RRC messagedepending on whether this RRC message includes a request message for theterminal (holding) information of the 5G UE 1 regarding the New 5G RAT(e.g., UE information request message) or depending on whether this RRCmessage includes a report message carrying the terminal (holding)information (e.g., UE information response message). That is, when anRRC message includes a report of (or a request for) the terminal(holding) information of the 5G UE 1 regarding the New 5G RAT, the 5G UE1 may transmit (or receive) this RRC message on any cell within the New5G CG. The base station system may receive (or transmit) this RRCmessage on any cell within the New 5G CG. In contrast, when an RRCmessage includes a report of (or a request for) the terminal (holding)information of the 5G UE 1 regarding the LTE RAT, the 5G UE 1 maytransmit (or receive) this RRC message on any LTE cell. Further, the 5GUE 1 may determine which one of the New 5G CG and the LTE CG is to beused to transmit an RRC message depending on the type of the terminal(holding) information to be requested or reported. The type of theterminal (holding) information may be any one or any combination of:

-   Radio link-related information;-   Random access-related information; and-   Mobility history-related information.

The radio link-related information may be information regardingdegradation of a radio quality (e.g., Radio Link Failure: RLF) in theLTE CG (e.g., PCell) or the New 5G CG (e.g., PSCell, Special cell) orinformation regarding a failure of radio link establishment (e.g., RRCConnection Establishment Failure: CEF, RRC Connection Re-establishmentfailure). For example, the information regarding degradation of a radioquality in the LTE CG may be requested or reported on the LTE CG (or theNew 5G CG). In a similar way, the information regarding the degradationof a radio quality in the 5G CG may be requested or reported on the New5G CG (or the LTE CG). In contrast, the information regarding a failureof radio link establishment may be always requested or reported on theLTE CG (or the New 5G CG). The radio link establishment may include aprocedure using an RRC Connection Reconfiguration to change theconfiguration of the radio protocol(s) (e.g., RRC, PDCP, RLC, MAC, orPHY). Further, the 5G UE 1 may store the information regarding a failureof radio link establishment in the LTE CG and the information regardinga failure of radio link establishment in the New 5G CG separately fromeach other.

The random access (RACH)-related information may be the number ofpreamble transmissions it has taken to achieve the success of the randomaccess and a flag indicating detection of a preamble contention. Forexample, the random access-related information regarding the LTE CG maybe requested or reported on the LTE CG (or the New 5G CG). In a similarway, the random access-related information regarding the New 5G CG maybe requested or reported on the New 5G CG (or on the LTE CG). The 5G UE1 may store the number of preamble transmissions on the LTE CG and thenumber of preamble transmissions on the New 5G CG separately from eachother.

The mobility history-related information (e.g., mobility history report)may be information regarding the cells which the 5G UE 1 visited (by thetime it receives a request to report this information) (e.g., visitedcell information list: VisitedCellInfoList). The visited cellinformation list may include at least one of: identification information(e.g., CGI, PCI, or Carrier frequency) of each cell that the 5G UE 1visited; and information (e.g., timeSpent) about the time (e.g.,seconds) the 5G UE 1 stayed in each cell. Further, the 5G UE 1 may storethe history of the specific New 5G cell (e.g., PSCell, Special cell) torecord information about visited cells specific to the New 5G RAT (e.g.,VisitedCellInfoList-5G, VisitedCellInfoList-SRAT). The visited cellinformation list regarding the LTE RAT may be requested or reported onthe LTE CG, and meanwhile the visited cell information list regardingthe New 5G RAT may be requested or reported on the 5G CG.

In a third implementation, the base station system may determine whichone of the New 5G CG and the LTE CG is to be used to transmit an RRCmessage depending on whether this RRC message includes the securityconfiguration information on the access stratum (AS) regarding one ormore New 5G cells within the New 5G CG. That is, the base station systemmay transmit an RRC message on any cell within the New 5G CG when thisRRC message includes the AS security configuration information regardingone or more New 5G cells. The 5G UE 1 may receive this RRC message onany cell within the New 5G CG. In contrast, the base station system maytransmit an RRC message on any LTE cell when this RRC message includesthe AS security configuration information regarding one or more LTEcells. The security configuration information may be transmitted as aNAS message (i.e., NAS PDU).

In a fourth implementation, the base station system may determine whichone of the New 5G CG and the LTE CG is to be used to transmit an RRCmessage depending on whether this RRC message includes theAS-security-activation request for one or more New 5G cells within theNew 5G CG. That is, the base station system may transmit an RRC messageon any cell within the New 5G CG when this RRC message includes theAS-security-activation request for one more New 5G cells (e.g., SecurityMode Command: SMC). The 5G UE 1 may receive this RRC message on any cellwithin the New 5G CG. In contrast, the base station system may transmitan RRC message on any LTE cell when this RRC message includes theAS-security-activation request for one or more LTE cells.

In a fifth implementation, the 5G UE 1 may determine which one of theNew 5G CG and the LTE CG is to be used to transmit an RRC messagedepending on whether this RRC message includes the response to theAS-security-activation request for one or more New 5G cells (e.g.,Security Mode Complete, Security Mode Failure). That is, the 5G UE 1 maytransmit an RRC message on any cell within the New 5G CG when this RRCmessage includes the response to the AS-security-activation request forone or more New 5G cells. The base station system may receive this RRCmessage on any cell within the New 5G CG. In contrast, the 5G UE 1 maytransmit an RRC message on any LTE cell when this RRC message includesthe response to the AS-security-activation request for one or more LTEcells.

In a sixth implementation, the base station system may determine whichone of the New 5G CG and the LTE CG is to be used to transmit an RRCmessage depending on whether this RRC message includes the configurationinformation specific to the New 5G RAT (e.g., SCG configuration for 5G,Secondary RAT (sRAT) configuration). That is, the base station systemmay transmit an RRC message on any cell within the New 5G CG when thisRRC message includes the configuration information specific to the New5G RAT. The 5G UE 1 may receive this RRC message on any cell within theNew 5G CG. In contrast, the base station system may transmit an RRCmessage on any LTE cell when this RRC message includes configurationinformation specific to the LTE RAT. The configuration informationspecific to the New 5G RAT may be any one or any combination of:

-   Terminal capability-related information;-   Service-related information; and-   5G CG management-related information.

The terminal capability-related information may be a terminalcapability(ies) regarding the New 5G RAT (e.g.,UE-EUTRA-CapabilityAddSRAT, UE-EUTRA-Capability-SRAT), or may be aterminal response to a request (or an inquiry) from the base stationsystem related to functions supported in a New 5G cell. The terminalcapability-related information may be requested (e.g., via UE CapabilityEnquiry message) and reported (e.g., via UE Capability Informationmessage) on the New 5G CG. In order to indicate that the terminalcapability-related information regarding the New 5G RAT is requested,the RAT-Type within the UE-CapabilityRequest IE may indicate “sRAT” or“eutra2”, for example. At least a part of the terminal capabilitiesregarding the New 5G RAT required to connect to a New 5G cell may betransmitted on a LTE cell.

The service-related information may be control information required toachieve a service(s) provided in the New 5G CG (e.g., Radio resourceconfiguration, Service area information, Service availabilityinformation, Service type/category information), or may be controlinformation regarding a content of a service(s) (e.g., Contentinformation, Scheduling/planning information). The service(s) here mayindicate a service(s) that can be provided also in LTE, or may be aservice(s) that can be provided only in 5G. The service(s) may include,for example, one of a Multimedia Broadcast and Multicast Service (MBMS)(i.e., MBMS over a Single Frequency Network (MBSFN), or single cellpoint to multipoint (SC-PTM)), a Cell Broadcast Service (CBS), ProximityServices (ProSe), Vehicle-to-Everything (V2X) services, and a MobileEdge Computing (MEC).

The 5G CG management-related information may be information regarding amodification of a specific cell (e.g., PSCell, Special cell) within theNew 5G CG (e.g., PSCell/Special cell reconfiguration,pSCellToAddMod-r1x). Further or alternatively, when the base stationsystem adds or releases another New 5G cell (e.g., 5G SCell) for the 5GUE 1 after the base station system configures the specific New 5G cell(e.g., PSCell, Special cell) for the 5G UE 1 (that is, after thespecific 5G cell becomes available), the base station system maytransmit radio resource configuration information about this other New5G cell (e.g., sCellToAddModListSRAT, sCellToReleaseListSRAT,RadioResourceConfigDedicatedSCellSRAT) as the 5G CG management-relatedinformation. That is, the information relating to addition/ release of acell within the New 5G CG and the information relating to modificationof the specific New 5G cell may be transmitted on any cell within theNew 5G CG.

Although basic configuration information required by the 5G UE 1 toconnect to (or detect) a New 5G cell is configuration informationspecific to the New 5G RAT, it may be transmitted on a LTE cell. Thisbasic configuration information may be, for example, informationregarding a subframe structure (which is different from that of LTE),information regarding a TTI length, information regarding a samplingrate, or information regarding a cell type.

In a sixth implementation, the base station system may determine whichone of the New 5G CG and the LTE CG is to be used to transmit an RRCmessage depending on whether this RRC message includes controlinformation newly defined by the 3GPP for New 5G cells. That is, thebase station system may transmit an RRC message on any cell within theNew 5G CG when this RRC message includes control information newlydefined for New 5G cells. The 5G UE 1 may receive this RRC message onany cell within the New 5G CG. In contrast, the base station system maytransmit an RRC message on any LTE cell when this RRC message includesexisting control information for LTE. The control information for New 5Gcells may be any one or a combination of:

-   Core network selection-related information; and-   Information regarding a standalone operation of a New 5G cell(s).

The core network selection-related information may be informationregarding a new type core network associated with introduction of theNew 5G RAT (e.g., the 5G specific EPC 42: this network is also referredto as a 5G Dedicated Core Network (DCN)). For example, the core networkselection-related information may be identification information (e.g.,MME ID, MMEC, GUMMEI, MMEGI, GUTI, TAI, or TEID) of a core network node(e.g., MME, S-GW, or P-GW), or may be a request (or a notification) forswitch of the connection with the core network node (e.g., RerouteCommand, Reroute Request, Reroute indication, DCN selection information,or DCN relocation information). Such information is required when theconnection is switched from the integrated EPC 41 to the 5G specific EPC42 or vice versa. Further or alternatively, the core networkselection-related information may be assistance information to selectthe 5G specific EPC (5G DCN) 42. This assistance information may be, forexample, a terminal type (e.g., device type), a usage type (e.g., UEusage type), or information about an expected (or preferred) corenetwork (e.g., expected/preferred CN information). The core networkselection-related information may be transmitted as a NAS message (i.e.,NAS PDU).

The information regarding the standalone operation of a New 5G cell(s)may be configuration information (e.g., Radio resource configuration for5G/SRAT PCell) that is required to use a New 5G cell as a primary cell(PCell) instead of using a New 5G cell as a secondary cell (SCell) withthe LTE PCell cell. Alternatively, the information regarding thestandalone operation of a New 5G cell(s) may be information (e.g.,Handover to a SG/SRAT cell or Redirection to a 5G/SRAT cell) indicatinga request or an indication that a New 5G cell is to be used as thePCell. Such information may be transmitted in a signalling radio bearer(e.g., SRBx) that will be newly defined. In addition, informationrequired to configure this SRBx may be transmitted on a New 5G cell oron a LTE cell.

In the above-described first to sixth implementations, the 5G UE 1 andthe base station system determine which one of the New 5G CG and the LTECG is to be used to transmit an RRC message depending on whether thisRRC message relates to the New 5G RAT (or a New 5G cell(s)).Accordingly, the 5G UE 1 and the base station system are able totransmit or receive RRC messages relating to the New 5G RAT (or a New 5Gcell(s)) through any cell within a New 5G CG. This, for example,contributes to transmission of RRC messages regarding the New 5G RAT (ora New 5G cell(s)) with low latency.

In a seventh implementation, the 5G UE 1 may determine which one of theNew 5G CG and the LTE CG is to be used to transmit a CP message (i.e.,an RRC message or a NAS message) depending on whether to transmit thisCP message via a specific signalling radio bearer associated with theNew 5G RAT. That is, the 5G UE 1 may transmit a CP message on any cellwithin the New 5G CG when the 5G UE 1 transmits this CP message via thespecific signalling radio bearer associated with the New 5G RAT. Thebase station system may receive this CP message on any cell within theNew 5G CG. The specific signalling radio bearer may be SRB0, SRB1, orSRB2 of LTE, or may be a new signalling radio bearer (e.g., SRBx). Incontrast, the 5G UE 1 may transmit a CP message on any LTE cell when the5G UE 1 transmits this CP message via a specific signalling radio bearerassociated with the LTE RAT.

In a similar way, in an eighth implementation, the base station systemmay determine which one of the New 5G CG and the LTE CG is to be used totransmit a CP message (i.e., an RRC message or a NAS message) dependingon whether to transmit this CP message via a specific signalling radiobearer associated with the New 5G RAT. That is, the base station systemmay transmit a CP message on any cell within the New 5G CG when the basestation system transmits this CP message via the specific signallingradio bearer associated with the New 5G RAT. The 5G UE 1 may receivethis CP message on any cell within the New 5G CG. In contrast, the basestation system may transmit a CP message on any LTE cell when the basestation system transmits this CP message via a specific signalling radiobearer associated with the LTE RAT.

In the above-described seventh and eighth implementations, the 5G UE 1and the base station system determine which one of the New 5G CG and theLTE CG is to be used to transmit a CP message depending on whether theCP message is transmitted on the specific bearer associated with the New5G RAT. Accordingly, the 5G UE 1 and the base station system are able totransmit or receive CP messages relating to the New 5G RAT (or the New5G cell(s)) through any cell within the New 5G CG. This, for example,contributes to transmission of CP messages regarding the New 5G RAT (ora New 5G cell(s)) with low latency.

In a ninth implementation, the 5G UE 1 may determine which one of theNew 5G CG and the LTE CG is to be used to transmit a CP message (i.e.,an RRC message or a NAS message) depending on whether the transmissioncause of this CP message indicates degradation of a radio quality of anyNew 5G cell. That is, the 5G UE 1 may transmit a CP message on any cellwithin the New 5G CG when this CP message is transmitted due todegradation of the radio quality of any New 5G cell. The base stationsystem may receive this CP message on any cell within the New 5G CG.This, for example, contributes to transmission of CP messages regardingthe New 5G RAT (or a New 5G cell(s)) with low latency. In contrast, the5G UE 1 may transmit an RRC message containing a NAS message on any LTEcell when the transmission cause of the CP message indicates degradationof a radio quality of any LTE cell.

In a tenth implementation, the 5G UE 1 may determine which one of theNew 5G CG and the LTE CG is to be used to transmit a NAS messagedepending on whether the core network associated with this NAS messageis a new type core network (e.g., the 5G specific EPC 42) associatedwith introduction of the New 5G RAT. That is, the 5G UE 1 may transmitan RRC message containing a NAS message on any cell within the New 5G CGwhen the core network associated with this NAS message is the new typecore network associated with introduction of the New 5G RAT. The basestation system may receive the RRC message containing this NAS messageon any cell within the New 5G CG. This, for example, contributes totransmission of NAS messages regarding the new type core network, whichis associated with introduction of the New 5G RAT, with low latency. Incontrast, the 5G UE 1 may transmit an RRC message containing a NASmessage on any LTE cell when the core network associated with this NASmessage is another core network (e.g., the integrated EPC 41).

In a similar way, in an eleventh implementation, the base station systemmay determine which one of the New 5G CG and the LTE CG is to be used totransmit a NAS message depending on whether the core network associatedwith this NAS message is a new-type core network (e.g., the 5G specificEPC 42) associated with introduction of the New 5G RAT. That is, thebase station system may transmit an RRC message containing a NAS messageon any cell within the New 5G CG when the core network associated withthis NAS message is the new type core network associated withintroduction of the New 5G RAT. The 5G UE 1 may receive the RRC messagecontaining this NAS message on any cell within the New 5G CG. Incontrast, the base station system may transmit an RRC message containinga NAS message on any LTE cell when the core network associated with thisNAS message is another core network (e.g., the integrated EPC 41).

In the aforementioned first to eleventh implementations, thetransmission or reception of RRC messages may be performed in a specificcell in the LTE CG or the New 5G CG. For example, when an RRC message istransmitted or received on the LTE CG, a primary cell (PCell) may beused. Alternatively, when an RRC message is transmitted or received onthe New 5G CG, a primary secondary cell (PSCell) may be used. The PCelland the PSCell may be collectively referred to as a special cell(SpCell).

In some implementations, the LTE+ eNB 5 may send, to the 5G specific eNB6, a CP message to be transmitted on a New 5G cell or informationregarding this message. Further or alternatively, the 5G specific eNB 6may send, to the LTE+ eNB 5, a CP message that is to be transmitted orhas been received (i.e., has been transmitted from the 5G UE 1) on a New5G cell or information regarding this message. Further, the 5G specificeNB 6 may send and receive control-plane information (i.e., a CP message(e.g., a NAS message, an S1 Application Protocol (SlAP) message)) to andfrom a node(s) (e.g., MME) in a core network (e.g., the integrated EPC41 or the 5G specific EPC 42).

FIG. 7 shows an example in which the LTE+ eNB 5 sends control-planeinformation (i.e., CP message) regarding a New 5G cell(s) to the 5Gspecific eNB 6. In Step 701A or 701B, the 5G specific eNB 6 receives anX2 message containing at least control-plane information (i.e., CPmessage) regarding a New 5G cell(s) from the LTE+ eNB 5. This X2 messagemay be a DL USER DATA message of the X2 User Plane Protocol (X2-UP)(Step 701A) or may be a DL CONTROL DATA message that will be newlydefined. This X2 message may be an X2 AP Message Transfer message of theX2AP (Step 701B). Alternatively, this X2 message may be an SeNBModification Request message (e.g., MeNB to SeNB Container) defined forDual Connectivity or may be an X2 message (e.g., SeNB ConfigurationRequest message) that will be newly defined.

Further or alternatively, the 5G specific eNB 6 may receive an S1APmessage containing control-plane information regarding a New 5G cell(s)from the integrated EPC 41 (e.g., the MME 7) or the 5G specific EPC 42(Step 702). This S1AP message may be an MME Configuration Transfermessage, an MME Direct Information Transfer message, or another S1APmessage. In Step 703, the 5G specific eNB 6 transmits an RRC ConnectionReconfiguration message containing the control-plane information (i.e.,CP message) regarding a New 5G cell(s), which has been received from theLTE+ eNB 5, the integrated EPC 41, or the 5G specific EPC 42, to the 5GUE 1 on a New 5G cell.

FIG. 8 shows an example in which the 5G specific eNB 6 sendscontrol-plane information (i.e., CP message) regarding a New 5G cell(s)to the LTE+ eNB 5. In Step 801A or 801B, the 5G specific eNB 6 sends anX2 message containing at least control-plane information (i.e., CPmessage) regarding a New 5G cell(s) to the LTE+ eNB 5. This X2 messagemay be an X2AP Message Transfer message (Step 801A). Alternatively, thisX2 message may be an SeNB Modification Required message defined for DualConnectivity (Step 801B).

The LTE+ eNB 5 may start an SeNB Modification procedure in response tothe reception of the X2 message. For example, the LTE+ eNB 5 may send anSeNB Modification Request message (containing an RRC Container (i.e.,SCG-ConfigInfo)) to the 5G specific eNB 6 (Step 802). In response to thereception of the SeNB Modification Request message, the 5G specific eNB6 may send an SeNB Modification Request Acknowledge message (containingan RRC Container (i.e., SCG-Config)) to the LTE+ eNB 5 (Step 803). InStep 803, the 5G specific eNB 6 may send control-plane informationregarding a 5G cell(s) to the LTE+ eNB 5.

The 5G specific eNB 6 may receive an S1AP message containingcontrol-plane information regarding a New 5G cell(s) from the integratedEPC 41 (e.g., the MME 7) or the 5G specific EPC 42 (Step 804). The 5Gspecific eNB 6 may perform Step 801A or 801B in response to receivingcontrol-plane information regarding a 5G cell(s) from the integrated EPC41 or the 5G specific EPC 42.

In Step 805, the 5G specific eNB 6 transmits an RRC ConnectionReconfiguration message containing the control-plane information (the CPmessage) regarding a New 5G cell(s) to the 5G UE 1 on a New 5G cell.

FIG. 9 shows another example in which the 5G specific eNB 6 sendscontrol-plane information (i.e., CP message) regarding a New 5G cell(s)to the LTE+ eNB 5. In Step 901, the 5G specific eNB 6 receivescontrol-plane information from the 5G UE 1 on a 5G cell. In Step 902,the 5G specific eNB 6 sends, to the LTE+ eNB 5, an X2 message containingat least a part of the control-plane information received from the 5G UE1. This X2 message may be an X2AP Message Transfer message, or may be anX2 message that will be newly defined (e.g., SeNB Configuration Updatemessage). The 5G specific eNB 6 may generate other control informationin response to receiving the control-plane information from the 5G UE 1and send the generated control information to the LTE+ eNB 5 in Step902.

Further, the 5G specific eNB 6 may send an S1AP message containing atleast a part of the control-plane information received from the 5G UE 1to the integrated EPC 41 (e.g., the MME 7) or the 5G specific EPC 42(Step 903). This S1AP message may be an eNB Configuration Transfermessage, an eNB Direct Information Transfer message, or another S1APmessage. The 5G specific eNB 6 may generate other control information inresponse to receiving the control-plane information from the 5G UE 1 andsend the generated control information to the integrated EPC 41 (e.g.,the MME 7) or the 5G specific EPC 42 in Step 903.

FIG. 10 is a flowchart showing one example (Process 1000) of theoperation of transmitting a CP message performed by the 5G UE 1 and thebase station system. In Step 1001, the RRC layer (i.e., the integratedRRC layer 401 or the New RRC layer 511) generates an RRC message. ThisRRC message may be an RRC message carrying a NAS message (e.g., a ULInformation Transfer message or a DL Information Transfer message).

In Step 1002, the RRC layer determines which one of the LTE CG and theNew 5G CG is to be used to transmit the RRC message depending on: (a) acontent or type of this RRC message; (b) a type of a signalling radiobearer used to transmit this RRC message; (c) a transmission cause ofthis RRC message; or (d) a type of a core network associated with theNAS message contained in this RRC message. As already described above,for example, when the content or type of this RRC message relates to theNew 5G RAT or a New 5G cell(s), the RRC layer may transmit this RRCmessage via any New 5G cell.

In Step 1003, the RRC layer sends this message to the PDCP entity thatcorresponds to the cell to be used for transmission of the RRC messagedetermined in Step 1002. Accordingly, the 5G UE 1 and the base stationsystem are able to control which one of the LTE CG and the New 5G CG isto be used to transmit an RRC message or a NAS message containedtherein.

Second Embodiment

The examples of a radio communication network and a radio protocol stackaccording to this embodiment are similar to those shown in FIGS. 1-6 .In this embodiment, selection of a key K_(eNB) to derive temporary keys(e.g., K_(UPenc), K_(RRCin)) used by each PDCP entity in the PDCP layer(i.e., the integrated PDCP layer 602 or the New PDCP layer 512) will beexplained. These temporary keys are used by each PDCP entity, forexample, to cipher and decipher the user plane (UP) traffic and the RRCtraffic. These temporary keys are derived from the key K_(eNB) by the 5GUE 1. In a similar way, these temporary keys are derived from the keyK_(eNB) by the integrated eNB 2, the LTE+ eNB 5, or the 5G specific eNB6.

In some implementations, the 5G UE 1 and the integrated eNB 2 (or theLTE+ eNB 5 and the 5G specific eNB 6) may use the first key K_(eNB) tocipher and decipher data of a radio bearer(s) of a certain bearer typeand use the second key sub-K_(eNB) to cipher and decipher data of aradio bearer(s) of another bearer type. The second key sub-K_(eNB) maybe derived from the first key K_(eNB), similar to the key S-K_(eNB) usedfor SCG bearers in Dual Connectivity (DC).

As shown in FIG. 11 , for example, the 5G UE 1 and the integrated eNB 2(or the LTE+ eNB 5 and the 5G specific eNB 6) may use the first keyK_(eNB) to cipher and decipher data of LTE bearers (e.g., the radiobearer #1 shown in FIG. 6 ) and integrated bearers (e.g., the radiobearer #3 shown in FIG. 6 ) and use the second key sub-K_(eNB) to cipherand decipher data of New 5G bearers (e.g., the radio bearer #2 shown inFIG. 6 ).

In the case of the Co-located deployments (FIGS. 1 and 2 ), for example,the 5G UE 1 and the integrated eNB 2 may use the first key K_(eNB) totransmit RRC messages through LTE bearers and use the same first keyK_(eNB) to transmit RRC messages through New 5G bearers. In contrast, inthe case of the non co-located deployments (FIG. 3 ), the 5G UE 1 andthe LTE+ eNB 5 may use the first key K_(eNB) to transmit RRC messagesthrough LTE bearers and, meanwhile, use the second key sub-K_(eNB) totransmit RRC messages through New 5G bearers.

In the following, configuration examples of the 5G UE 1, the integratedeNB 2, the LTE+ eNB 5, and the 5G specific eNB 6 according to the aboveembodiments will be described. FIG. 12 is a block diagram showing aconfiguration example of the 5G UE 1. An LTE transceiver 1201 performsanalog RF signal processing regarding the PHY layer of the LTE RAT tocommunicate with the integrated eNB 2 (or the LTE+ eNB 5). The analog RFsignal processing performed by the LTE transceiver 1301 includesfrequency up-conversion, frequency down-conversion, and amplification.The LTE transceiver 1201 is coupled to an antenna 1202 and a basebandprocessor 1205. That is, the LTE transceiver 1201 receives modulatedsymbol data (or OFDM symbol data) from the baseband processor 1205,generates a transmission RF signal, and supplies the transmission RFsignal to the antenna 1202. Further, the LTE transceiver 1201 generatesa baseband reception signal based on a reception RF signal received bythe antenna 1202, and supplies the baseband reception signal to thebaseband processor 1205.

A New 5G transceiver 1203 performs analog RF signal processing regardingthe PHY layer of the New 5G RAT r to communicate with the integrated eNB2 (or the 5G specific eNB 6). The New 5G transceiver 1203 is coupled toan antenna 1204 and the baseband processor 1205.

The baseband processor 1205 performs digital baseband signal processing(data-plane processing) and control-plane processing for radiocommunication. The digital baseband signal processing includes (a) datacompression/decompression, (b) data segmentation/concatenation, (c)composition/decomposition of a transmission format (i.e., transmissionframe), (d) channel coding/decoding, (e) modulation (i.e., symbolmapping)/demodulation, and (f) generation of OFDM symbol data (i.e.,baseband OFDM signal) by Inverse Fast Fourier Transform (IFFT).Meanwhile, the control-plane processing includes communicationmanagement of layer 1 (e.g., transmission power control), layer 2 (e.g.,radio resource management and hybrid automatic repeat request (HARQ)processing), and layer 3 (e.g., signalling regarding attach, mobility,and packet communication).

In the case of LTE and LTE-Advanced, for example, the digital basebandsignal processing performed by the baseband processor 1205 may includesignal processing of a Packet Data Convergence Protocol (PDCP) layer, aRadio Link Control (RLC) layer, a MAC layer, and a PHY layer. Further,the control-plane processing performed by the baseband processor 1205may include processing of a Non-Access Stratum (NAS) protocol, an RRCprotocol, and MAC CEs.

The baseband processor 1205 may include a modem processor (e.g., aDigital Signal Processor (DSP)) that performs the digital basebandsignal processing and a protocol stack processor (e.g., a CentralProcessing Unit (CPU) or a Micro Processing Unit (MPU)) that performsthe control-plane processing. In this case, the protocol stackprocessor, which performs the control-plane processing, may beintegrated with an application processor 1206 described in thefollowing.

The application processor 1206 is also referred to as a CPU, an MPU, amicroprocessor, or a processor core. The application processor 1206 mayinclude a plurality of processors (processor cores). The applicationprocessor 1206 loads a system software program (Operating System (OS))and various application programs (e.g., communication application thatacquires metering data or sensing data) from a memory 1208 or a memory(not shown) and executes these programs, thereby providing variousfunctions of the 5G UE 1.

In some implementations, as represented by a dashed line (1207) in FIG.12 , the baseband processor 1205 and the application processor 1206 maybe integrated on a single chip. In other words, the baseband processor1205 and the application processor 1206 may be implemented in a singleSystem on Chip (SoC) device 1207. An SoC device may be referred to as asystem Large Scale Integration (LSI) or a chipset.

The memory 1208 is a volatile memory, a non-volatile memory, or acombination thereof. The memory 1208 may include a plurality of memorydevices that are physically independent from each other. The volatilememory is, for example, a Static Random Access Memory (SRAM), a DynamicRAM (DRAM), or a combination thereof. The non-volatile memory is, forexample, a mask Read Only Memory (MROM), an Electrically ErasableProgrammable ROM (EEPROM), a flash memory, a hard disc drive, or anycombination thereof. The memory 1208 may include, for example, anexternal memory device that can be accessed from the baseband processor1205, the application processor 1206, and the SoC 1207. The memory 1208may include an internal memory device that is integrated in the basebandprocessor 1205, the application processor 1206, or the SoC 1207.Further, the memory 1208 may include a memory in a Universal IntegratedCircuit Card (UICC).

The memory 1208 may store one or more software modules (computerprograms) 1209 including instructions and data to perform the processingby the 5G UE 1 described in the above embodiments. In someimplementations, the baseband processor 1205 or the applicationprocessor 1206 may load these software modules 1209 from the memory 1208and execute the loaded software modules, thereby performing theprocessing of the 5G UE 1 described in the above embodiments.

FIG. 13 is a block diagram showing a configuration example of theintegrated eNB 2 according to the above embodiments. Referring to FIG.13 , the eNB 2 includes an LTE transceiver 1301, a New 5G transceiver1303, a network interface 1305, a processor 1306, and a memory 1307. TheLTE transceiver 1301 performs analog RF signal processing regarding thePHY layer of the LTE RAT to communicate with the 5G UE 1 via an LTEcell. The LTE transceiver 1301 may include a plurality of transceivers.The LTE transceiver 1301 is coupled to an antenna 1302 and the processor1306.

The New 5G transceiver 1303 performs analog RF signal processingregarding the PHY layer of the New 5G RAT to communicate with the 5G UE1 via a New 5G cell. The New 5G transceiver 1303 is coupled to anantenna 1304 and the baseband processor 1306.

The network interface 1305 is used to communicate with a network node inthe integrated EPC 41 or the 5G specific EPC 42 (e.g., a MobilityManagement Entity (MME) and a Serving Gateway (S-GW)), and tocommunicate with other eNBs. The network interface 1305 may include, forexample, a network interface card (NIC) conforming to the IEEE 802.3series.

The processor 1306 performs digital baseband signal processing(data-plane processing) and control-plane processing for radiocommunication. In the case of LTE and LTE-Advanced, for example, thedigital baseband signal processing performed by the processor 1306 mayinclude signal processing of the PDCP layer, the RLC layer, the MAClayer, and the PHY layer. Further, the control-plane processingperformed by the processor 1306 may include processing of the S1protocol, the RRC protocol, and MAC CEs.

The processor 1306 may include a plurality of processors. The processor1306 may include, for example, a modem processor (e.g., DSP) thatperforms the digital baseband signal processing and a protocol stackprocessor (e.g., a CPU or an MPU) that performs the control-planeprocessing.

The memory 1307 is composed of a combination of a volatile memory and anon-volatile memory. The volatile memory is, for example, an SRAM, aDRAM, or a combination thereof. The non-volatile memory is, for example,an MROM, a PROM, a flash memory, a hard disc drive, or a combinationthereof. The memory 1307 may include a storage located apart from theprocessor 1306. In this case, the processor 1306 may access the memory1307 via the network interface 1305 or an I/O interface (not shown).

The memory 1307 may store a software module(s) (computer program) 1308including instructions and data for performing processing by theintegrated eNB 2 described in the above embodiments. In someimplementations, the processor 1306 may be configured to load thesoftware module(s) 1308 from the memory 1307 and execute the loadedsoftware module(s), thereby performing processing of the integrated eNB2 described in the above embodiments.

The configurations of the LTE+ eNB 5 and the 5G specific eNB 6 may besimilar to the configuration of the integrated eNB 2 shown in FIG. 13 .However, the LTE+ eNB 5 does not need to include the New 5G transceiver1303 and the 5G specific eNB 6 does not need to include the LTEtransceiver 1301.

As described above with reference to FIGS. 12 and 13 , each of theprocessors included in the 5G UE 1, the integrated eNB 2, the LTE+ eNB5, and the 5G specific eNB 6 according to the aforementioned embodimentsexecutes one or more programs including instructions to cause a computerto perform an algorithm described with reference to the drawings. Theprogram(s) can be stored and provided to a computer using any type ofnon-transitory computer readable media. Non-transitory computer readablemedia include any type of tangible storage media. Examples ofnon-transitory computer readable media include magnetic storage media(such as flexible disks, magnetic tapes, hard disk drives, etc.),optical magnetic storage media (e.g., magneto-optical disks), CompactDisc Read Only Memory (CD-ROM), CD-R, CD-R/W, and semiconductor memories(such as mask ROM, Programmable ROM (PROM), Erasable PROM (EPROM), flashROM, Random Access Memory (RAM), etc.). The program(s) may be providedto a computer using any type of transitory computer readable media.Examples of transitory computer readable media include electric signals,optical signals, and electromagnetic waves. Transitory computer readablemedia can provide the program to a computer via a wired communicationline (e.g., electric wires, and optical fibers) or a wirelesscommunication line.

Other Embodiments

Each of the above-described embodiments may be used individually, or twoor more embodiments may be appropriately combined with one another.

The protocol stacks described in the above embodiments are merelyexamples and another protocol stack may be used to achieve interworkingof the LTE RAT and the New 5G RAT. In some implementations, the existingprotocol stacks for LTE/LTE-Advanced Carrier Aggregation (CA) or DualConnectivity (DC), or any modification thereof may be used to achieveinterworking of the LTE RAT and the New 5G RAT. In the co-locateddeployments or the co-located RAN, for example, an integrated MAC layer(or sublayer) may be used in place of the integrated PDCP layer (orsublayer). In this case, the integrated MAC layer may control the LTEPHY layer and the New PHY layer and perform CA using the LTE cell andthe New 5G cell.

The base station, the base station system, the Integrated eNB 2, theLTE+ eNB 5, the 5G specific eNB 6, the BBU (or the DU), and the RRH (orthe RU) described in the aforementioned embodiments may be each referredto as a radio station or a radio access network (RAN) node. In otherwords, the processing and the operations performed by the base station,the base station system, the Integrated eNB 2, the LTE+ eNB 5, the 5Gspecific eNB 6, the BBU (DU), or the RRH (RU) described in the aboveembodiments may be provided by any one or more radio stations (i.e., RANnodes).

Further, the above-described embodiments are merely examples ofapplications of the technical ideas obtained by the inventors. Needlessto say, these technical ideas are not limited to the above-describedembodiments and various modifications can be made thereto.

For example, the whole or part of the embodiments disclosed above can bedescribed as, but not limited to, the following supplementary notes.

(Supplementary Note 1)

A radio station system comprising:

-   one or more radio stations configured to:    -   simultaneously provide, for one radio terminal, at least one        first cell in accordance with a first radio access technology        and at least one second cell in accordance with a second radio        access technology and used in addition and subordinate to the at        least one first cell to one radio terminal; and    -   transmit or receive a control-plane message to or from the radio        terminal on the at least one second cell when a predetermined        condition is satisfied, wherein-   the control-plane message includes a non-access stratum (NAS)    message or a radio resource control (RRC) message or both, and-   the predetermined condition relates to at least one of: (a) a    content or type of the control-plane message; (b) a type of a    signalling radio bearer used to transmit the control-plane    message; (c) a transmission cause of the control-plane message;    and (d) a type of a core network associated with the NAS message.

(Supplementary Note 2)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the one or more radio stations are configured to transmit or receive    the control-plane message on the at least one second cell when the    control-plane message comprises: a request for terminal measurement    regarding the at least one second cell sent to the radio terminal;    or a report on a measurement result regarding the at least one    second cell sent from the radio terminal.

(Supplementary Note 3)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the one or more radio stations are configured to transmit or receive    the control-plane message on the at least one second cell when the    control-plane message comprises: a request for terminal information    of the radio terminal regarding the second radio access technology    sent to the radio terminal; or a report on the terminal information    sent from the radio terminal.

(Supplementary Note 4)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the one or more radio stations are configured to transmit the    control-plane message on the at least one second cell when the    control-plane message comprises security configuration information    about an access stratum (AS) for the at least one second cell.

(Supplementary Note 5)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the one or more radio stations are configured to transmit or receive    the control-plane message on the at least one second cell when the    control-plane message comprises: a request for access-stratum    (AS)-security activation for the at least one second cell sent to    the radio terminal; or a response to the request from the radio    terminal.

(Supplementary Note 6)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the one or more radio stations are configured to transmit the    control-plane message on the at least one second cell when the    control-plane message includes configuration information specific to    the second radio access technology.

(Supplementary Note 7)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to the type of the signalling    radio bearer used to transmit the control-plane message, and-   the one or more radio stations are configured to transmit or receive    the control-plane message on the at least one second cell when the    control-plane message is transmitted via a specific signalling radio    bearer associated with the second radio access technology.

(Supplementary Note 8)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to a transmission cause of the    control-plane message, and-   the one or more radio stations are configured to receive the    control-plane message on the at least one second cell when the    control-plane message is transmitted due to degradation of a radio    quality of the at least one second cell.

(Supplementary Note 9)

The radio station system according to Supplementary Note 1, wherein

-   the predetermined condition relates to the type of the core network    associated with the NAS message, and-   the one or more radio stations are configured to transmit or receive    the control-plane message containing the NAS message on the at least    one second cell when the core network is a new type core network    associated with introduction of the second radio access technology.

(Supplementary Note 10)

The radio station system according to any one of Supplementary Notes 1to 9, wherein the one or more radio stations are configured to provideRadio Link Control (RLC) and Medium Access Control (MAC) layers tocommunicate with the radio terminal in accordance with the first radioaccess technology, Radio Link Control (RLC) and Medium Access Control(MAC) layers to communicate with the radio terminal in accordance withthe second radio access technology, a common Packet Data ConvergenceProtocol (PDCP) layer associated with both of the two RLC layers, and acommon RRC layer associated with the common PDCP layer.

(Supplementary Note 11)

The radio station system according to any one of Supplementary Notes 1to 9, wherein

-   the one or more radio stations comprise a first radio station for    the first radio access technology and a second radio station for the    second radio access technology,-   the second radio station provides Radio Link Control (RLC) and    Medium Access Control (MAC) layers to communicate with the radio    terminal in accordance with the second radio access technology, and-   the first radio station is configured to provide Radio Link Control    (RLC) and Medium Access Control (MAC) layers to communicate with the    radio terminal in accordance with the first radio access technology,    a common Packet Data Convergence Protocol (PDCP) layer associated    with both of the two RLC layers, and a common RRC layer associated    with the common PDCP layer.

(Supplementary Note 12)

The radio station system according to any one of Supplementary Notes 1to 11, wherein

-   the first radio access technology is continuous enhancement of LTE    and LTE-Advanced, and-   the second radio access technology is a new 5G radio access    technology.

(Supplementary Note 13)

A method in a radio station system including one or more radio stations,the method comprising:

-   simultaneously providing, for one radio terminal, at least one first    cell in accordance with a first radio access technology and at least    one second cell in accordance with a second radio access technology    and used in addition and subordinate to the at least one first cell;    and-   transmitting or receiving a control-plane message to or from the    radio terminal on the at least one second cell when a predetermined    condition is satisfied, wherein    -   the control-plane message includes a non-access stratum (NAS)        message or a radio resource control (RRC) message or both, and    -   the predetermined condition relates to at least one of: (a) a        content or type of the control-plane message; (b) a type of a        signalling radio bearer used to transmit the control-plane        message; (c) a transmission cause of the control-plane message;        and (d) a type of a core network associated with the NAS        message.

(Supplementary Note 14)

A radio terminal comprising:

-   a memory; and-   at least one processor coupled to the memory and configured to:    -   communicate with a radio station system comprising one or more        radio stations simultaneously on at least one first cell in        accordance with a first radio access technology and at least one        second cell in accordance with a second radio access technology        and used in addition and subordinate to the at least one first        cell; and    -   transmit or receive a control-plane message to or from the radio        station system on the at least one second cell when a        predetermined condition is satisfied, wherein-   the control-plane message includes a non-access stratum (NAS)    message or a radio resource control (RRC) message or both, and-   the predetermined condition relates to at least one of: (a) a    content or type of the control-plane message; (b) a type of a    signalling radio bearer used to transmit the control-plane    message; (c) a transmission cause of the control-plane message;    and (d) a type of a core network associated with the NAS message.

(Supplementary Note 15)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the at least one processor is configured to receive or transmit the    control-plane message on the at least one second cell when the    control-plane message comprises: a request for terminal measurement    regarding the at least one second cell sent to the radio terminal;    or a report on a measurement result regarding the at least one    second cell sent from the radio terminal.

(Supplementary Note 16)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the at least one processor is configured to receive or transmit the    control-plane message on the at least one second cell when the    control-plane message comprises: a request for terminal information    of the radio terminal regarding the second radio access technology    sent to the radio terminal; or a report on the terminal information    sent from the radio terminal.

(Supplementary Note 17)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the at least one processor is configured to receive the    control-plane message on the at least one second cell when the    control-plane message comprises security configuration information    about an access stratum (AS) for the at least one second cell.

(Supplementary Note 18)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the at least one processor is configured to receive or transmit the    control-plane message on the at least one second cell when the    control-plane message comprises: a request for access-stratum    (AS)-security activation for the at least one second cell sent to    the radio terminal; or a response to the request from the radio    terminal.

(Supplementary Note 19)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to the content or type of the    control-plane message, and-   the at least one processor is configured to receive the    control-plane message on the at least one second cell when the    control-plane message comprises configuration information specific    to the second radio access technology.

(Supplementary Note 20)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to the type of the signalling    radio bearer used to transmit the control-plane message, and-   the at least one processor is configured to receive or transmit the    control-plane message on the at least one second cell when the    control-plane message is transmitted via a specific signalling radio    bearer associated with the second radio access technology.

(Supplementary Note 21)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to a transmission cause of the    control-plane message, and-   the at least one processor is configured to transmit the    control-plane message on the at least one second cell when the    control-plane message is transmitted due to degradation of a radio    quality of the at least one second cell.

(Supplementary Note 22)

The radio terminal according to Supplementary Note 14, wherein

-   the predetermined condition relates to the type of the core network    associated with the NAS message, and-   the at least one processor is configured to receive or transmit the    control-plane message containing the NAS message on the at least one    second cell when the core network is a new type core network    associated with introduction of the second radio access technology.

(Supplementary Note 23)

The radio terminal according to any one of Supplementary Notes 14 to 22,wherein the at least one processor is configured to provide Radio LinkControl (RLC) and Medium Access Control (MAC) layers to communicate withthe radio station system in accordance with the first radio accesstechnology, Radio Link Control (RLC) and Medium Access Control (MAC)layers to communicate with the radio station system in accordance withthe second radio access technology, a common Packet Data ConvergenceProtocol (PDCP) layer associated with both of the two RLC layers, and acommon RRC layer associated with the common PDCP layer.

(Supplementary Note 24)

The radio terminal according to any one of Supplementary Notes 14 to 23,wherein

-   the first radio access technology is continuous enhancement of LTE    and LTE-Advanced, and-   the second radio access technology is a new 5G radio access    technology.

(Supplementary Note 25)

A method in a radio terminal, the method comprising:

-   communicating with a radio station system comprising one or more    radio stations simultaneously on at least one first cell in    accordance with a first radio access technology and at least one    second cell in accordance with a second radio access technology and    used in addition and subordinate to the at least one first cell; and-   transmitting or receiving a control-plane message to or from the    radio station system on the at least one second cell when a    predetermined condition is satisfied, wherein    -   the control-plane message includes a non-access stratum (NAS)        message or a radio resource control (RRC) message or both, and    -   the predetermined condition relates to at least one of: (a) a        content or type of the control-plane message; (b) a type of a        signalling radio bearer used to transmit the control-plane        message; (c) a transmission cause of the control-plane message;        and (d) a type of a core network associated with the NAS        message.

(Supplementary Note 26)

A non-transitory computer readable medium storing a program for causinga computer to perform a method in a radio terminal, the methodcomprising:

-   communicating with a radio station system comprising one or more    radio stations simultaneously on at least one first cell in    accordance with a first radio access technology and at least one    second cell in accordance with a second radio access technology and    used in addition and subordinate to the at least one first cell; and-   transmitting or receiving a control-plane message to or from the    radio station system on the at least one second cell when a    predetermined condition is satisfied, wherein    -   the control-plane message includes a non-access stratum (NAS)        message or a radio resource control (RRC) message or both, and    -   the predetermined condition relates to at least one of: (a) a        content or type of the control-plane message; (b) a type of a        signalling radio bearer used to transmit the control-plane        message; (c) a transmission cause of the control-plane message;        and (d) a type of a core network associated with the NAS        message.

Reference Signs List 1 RADIO TERMINAL (5G UE) 2 BASE STATION (INTEGRATEDeNB) 3 RRH 5 LTE+ eNB 6 5G SPECIFIC eNB 41 INTEGRATED EPC 42 5G SPECIFICEPC 1201 LTE TRANSCEIVER 1203 NEW 5G TRANSCEIVER 1205 BASEBAND PROCESSOR1206 APPLICATION PROCESSOR 1208 MEMORY 1301 LTE TRANSCEIVER 1303 NEW 5GTRANSCEIVER 1306 PROCESSOR 1307 MEMORY

1. A method in a radio station system including one or more radiostations, the method comprising: providing, for a radio terminal, one ormore first cells in accordance with a first radio access technology of afirst radio station and one or more second cells in accordance with asecond radio access technology of a second radio station, the one ormore second cells being used in addition to the one or more first cells,wherein the first radio station and the second radio station use anintegrated Medium Access Control (MAC) layer, and the integrated MAClayer has a function related to transmitting and receiving at least onetransport block between the integrated MAC layer and a first physical(PHY) layer in accordance with the first radio access technology, andtransmitting and receiving at least one transport block between theintegrated MAC layer and a second physical (PHY) layer in accordancewith the second radio access technology, wherein the integrated MAClayer transmits and receives a data by using a Carrier Aggregation (CA)function using the one or more first cells and the one or more secondcells or a technology improving the CA function.
 2. The method accordingto claim 1, wherein the first radio station includes the integrated MAClayer and the first PHY layer, and the second radio station includes thesecond PHY layer.
 3. The method according to claim 1, wherein theintegrated MAC layer is a single MAC entity.
 4. The method according toclaim 2, wherein the integrated MAC layer is a single MAC entity.
 5. Aradio station system comprising: a first radio station configured to:provide, for a radio terminal, one or more first cells in accordancewith a first radio access technology of the first radio station, and asecond radio station configured to: provide, for the radio terminal, oneor more second cells in accordance with a second radio access technologyof the second radio station, the one or more second cells being used inaddition to the one or more first cells, wherein the first radio stationand the second radio station use an integrated Medium Access Control(MAC) layer, and the integrated MAC layer has a function related totransmitting and receiving at least one transport block between theintegrated MAC layer and a first physical (PHY) layer in accordance withthe first radio access technology, and transmitting and receiving atleast one transport block between the integrated MAC layer and a secondphysical (PHY) layer in accordance with the second radio accesstechnology, wherein the integrated MAC layer transmits and receives adata by using a Carrier Aggregation (CA) function using the one or morefirst cells and the one or more second cells or a technology improvingthe CA function.
 6. A method in a radio terminal, the method comprising:communicating with a radio station system comprising one or more radiostations on one or more first cells in accordance with a first radioaccess technology and one or more second cells in accordance with asecond radio access technology, the one or more second cells being usedin addition to the one or more first cells; and using an integratedMedium Access Control (MAC) layer, wherein the integrated MAC layer hasa function related to transmitting and receiving at least one transportblock between the integrated MAC layer and a first physical (PHY) layerin accordance with the first radio access technology, and transmittingand receiving at least one transport block between the integrated MAClayer and a second physical (PHY) layer in accordance with the secondradio access technology, wherein the integrated MAC layer transmits andreceives a data by using a Carrier Aggregation (CA) function using theone or more first cells and the one or more second cells or a technologyimproving the CA function.
 7. The method according to claim 6, whereinthe common MAC layer is a single MAC entity.
 8. A radio terminalcomprising: a memory; and at least one processor coupled to the memoryand configured to: communicate with a radio station system comprisingone or more radio stations on one or more first cells in accordance witha first radio access technology and one or more second cells inaccordance with a second radio access technology, the one or more secondcells being used in addition to the one or more first cells; and use anintegrated Medium Access Control (MAC) layer, wherein the integrated MAClayer has a function related to transmitting and receiving at least onetransport block between the integrated MAC layer and a first physical(PHY) layer in accordance with the first radio access technology, andtransmitting and receiving at least one transport block between theintegrated MAC layer and a second physical (PHY) layer in accordancewith the second radio access technology, wherein the integrated MAClayer transmits and receives a data by using a Carrier Aggregation (CA)function using the one or more first cells and the one or more secondcells or a technology improving the CA function.